1. Field of the Invention
This invention relates generally to optical data storage media and systems and more particularly to a write once magneto-optic media and system.
2. Description of the Prior Art
Optical data storage systems store large quantities of data on a single optical disk medium. Write once read many (WORM) and erasable magneto-optic (MO) are two types of optical systems.
WORM systems are used to permanently record data. Specific applications include storing the financial data of financial institutions and claim documentation of insurance companies. The WORM systems typically use a media upon which data is permanently recorded by burning pits into the media or changing the phase of spots on the media with a laser beam at relatively high power. The data is then read by directing a low power laser beam at the media. The data pits or spots have different reflectivity than the surrounding media and this difference in reflectivity is detected by the optical channel and converted into a digital signal representative of the recorded data.
MO systems allow data to be written and erased repeatedly from the same media. The systems use a media which contains rare earth-transition metal compositions. Initially, the media has its magnetic domain oriented in a first direction. Data is recorded by directing a high power laser beam to a spot on the media in order to heat the media above its Curie temperature. At approximately this temperature, the media has negligible magnetic coercivity and its magnetic domain is easily oriented in a second direction by a small biasing electro-magnet in the disk drive. When the material cools below its Curie temperature it retains its magnetic orientation. The data is then read by directing a low power polarized laser beam at the media. The different magnetic domain orientations cause the laser beam's plane of polarization to be rotated one way or the other upon reflecting from the media. This is known as the Kerr effect. This difference in polarization is detected by the optical channel and a data signal representative of the recorded data is generated.
The MO media may be erased by re-heating the media to above its Curie temperature with the laser and then reversing the magnetic domain with the small biasing magnet. A disadvantage of the scheme is that the media must be rotated one complete revolution to erase a track on the media before re-writing may begin. Various schemes to directly overwrite (DOW) the MO media had been proposed. These include Saito, et al. "Direct Overwrite By Light Power Modulation On Magneto-Optical Multi-Layered Media," Japanese Journal of Applied Physics, Vol. 26 (1987), supplement 26-4; European Patent Application 89313591.3, filed Dec. 27, 1989 by Shiratori; European Patent Application 90302213.5, filed Jan. 3, 1990, by Matsumoto; Japanese application 01-128246, filed May 19, 1989 by Yumoto, et al.; Japanese application 63-237242, filed Oct. 3, 1988 by Kobayashi, et al.; and Japanese application 01-237946, filed Sep. 22, 1989 by Inoue, et al.
Optical disk drive users need the flexibility of being able to use both WORM and erasable MO media as their applications dictate. However, the standard WORM media are not compatible with standard MO drives and visa versa. It is desirable to build a single drive which can handle both WORM and MO. However, with the present WORM media such a drive would have to be able to do both reflectivity and polarization detection. This would necessitate two different optical channels and some way for the drive to identify which media it is using and switch between the channels. The result is a complex drive which is not optimized for either MO or WORM.